CENTRIFUGAL PUMP SELECTION. HOW TO
PICK THE CORRECT SIZE PUMP FOR YOUR APPLICATION.

The following paper is an excerpt from my
book "Bill
Mc Nally's Centrifugal Pump And Mechanical Seal Reference
Manual". The words marked in
blue
reference you to the alphabetical section of the book or directly to
the subject on the CD
for a detailed explanation of that particular topic. If you do not
have a copy of the book or CD you can find a lot of the information
in the individual papers I have published on the web.Check out the
indexfor a list of my papers by
subject.

We will begin by deciding what operating conditions our pump has
to meet and then we will approach pump suppliers to see how closely
they can satisfy these needs. Unfortunately no comprehensive theory
which would permit the complete hydrodynamic design of a centrifugal
pump has evolved in the many years that pumps have been around, so
the pump manufacturer will be doing the best he can with the
information you supply to him.

To clearly define the capacity and
pressure needs of our system we will
construct a type of graph called a system
curve. This system curve will then be given to the pump
suppliers and they will try to match it with a
pump curve that satisfies these needs as
closely as possible.

To start the construction of the system curve I will assume you
want to pump some fluid from point "A" to point "B". To do that
efficiently you must make a couple of decisions:

Decide the capacity you will need. This means the gallons per
minute or cubic meters per hour. You must also consider if this
capacity will change with the operation of your process. A boiler
feed pump is an example of an application that needs a constant
pressure with varying capacities to meet a changing steam demand.
The demand for boiler water is regulated by opening and closing a
control valve on the discharge side of the pump with a discharge
re-circulation line returning the unneeded portion back to a
convenient storage place or the suction side of the pump. Remember
that with a centrifugal pump if you change its capacity you change
the pressure also. A rotary or positive
displacement pump is different. It puts out a constant
capacity regardless of the pressure.

For other centrifugal pump applications, you are going to have
to calculate how much pressure will be needed to deliver different
capacities to the point where you will need them. You will need
enough pressure to :

Reach the maximum static head
or height the fluid will have to attain.

Enough discharge pressure to over come any pressure that
might be in the vessel where the fluid is discharging, such as
the boiler we just discussed. This is called the
pressure head.

Overcome friction resistance in the lines, fittings and any
valves or hardware that might be in the system. As an example:
high-pressure nozzles can be tricky, especially if they clog
up. This resistance is called the
friction head.

Will you need any special materials for the pump
components?

The pump manufacturer will try to choose pump metal components
that are chemically compatible with what you are pumping as well
as any cleaners or solvents that might be flushed through the
lines. If the temperature of the
pumpage changes the corrosion rate
can change also. His choice of materials could have a serious
affect on your spare parts inventory. Will he be selecting
universal and easily obtainable materials? Unless you have a great
deal of experience with the product you are pumping do not select
the metal components by using a compatibility chart. Metal
selection is a job for metallurgists.

If the product you are pumping is explosive or a fire hazard,
you should be looking at non-sparking materials for the pump
components. Do not depend totally upon the pump manufacturer to
make this decision for you. If you are not sure what materials are
compatible with your product, how will the pump man know? Also,
keep in mind that some of the fluids you will be pumping could be
proprietary products known only by their trade name.

Dangerous and radioactive materials will dictate special
materials.

Food products require high-density seal and pump materials
that are easy to clean.

If there are abrasive solids in the pumpage you will need
materials with good wearing capabilities. Hard surfaces and
chemically resistant materials are often incompatible. You may
have to go to some type of coating on the pump wetted parts or
select an expensive duplex
metal.

Occasionally you will find an application where metal is
either not compatible or not practical. There are many monomer and
polymer materials available for these applications, but their cost
is generally higher than comparable metal parts. Be aware that if
you are using a mechanical seal in a non-metallic pump, the seal
cannot have metal parts in contact with the fluid for the same
reasons the pump was manufactured from non-metallic
materials.

Since we are just getting into the subject, one of the first
things we should learn is that centrifugal pump people do not use the
word pressure. As mentioned in an earlier paragraph they substitute
the word head, so you will have to calculate the three kinds of head
that will be combined together to give you the total head of the
system required to deliver the needed capacity. Here are the three
kinds of head you will be calculating:

The static head or maximum height that the liquid will reach.
We must also learn how to compensate for the
siphon affect from down running pipes
on the discharge side of the pump. Remember that if you fill a
tank from the bottom instead of the top the static head will
continually increase. This is not a good application for a
centrifugal pump because the capacity is decreasing with an
increasing head. If you must fill from the bottom, or if you will
be using the pump as an accumulator, a
rotary positive displacement pump
will be your best choice as long as it can meet the needed
capacities.

The pressure heads are next if the container we are pumping to
or from is pressurized. We will have to learn how to
convert pressure units to head units
because later on we will need this conversion knowledge to read
the manufacturers pump curve. Pump gages are labeled in psi or
bar, pump curves are printed in feet of head, or meters of
head.

The friction head is the last one that we will have to
calculate. This head tells us how much friction or resistance head
there is in both the suction and discharge piping along with the
fittings and valves in the piping system. And to make the job a
little tougher this head changes dramatically as the pump capacity
changes.

You will be calculating these heads on both the suction and
discharge side of the pump. To get the total head you will subtract
the suction head from the discharge head and that will be the head
that the pump must produce to satisfy the application. It will become
obvious in the calculations, but I should mention here that if the
suction head is a negative number, the suction and discharge heads
will be added together to get the total head. If you subtract a minus
number from a positive number you must add the numbers together. As
an example: 4 - (-2) = + 6

The total head of a pump seldom remains static. There are a number
of factors that can change the head of a pump while it is operating
and you should be familiar with most of them. You can find these
factors in the alphabetical section of my book labeled
"head, the reasons for changes in the discharge
head of a centrifugal pump".

All of this head information is calculated from charts and graphs
you will find in the appendix and articles in the alphabetical
section of the manual and on these web pages. This head data will be
plotted on a set of coordinates called a system
curve. Since we will not be operating at a single point all of
the time we will make the calculations for a range of different
capacities and heads that we might expect to encounter. This range is
described as the operating window we
will need to satisfy the application.

Making these calculations is not an exact science because the
piping is seldom new, pipe inside diameters are not exact and the
charts and graphs you will be consulting cannot compensate for
corrosion and solids built up on the piping, valve and fitting walls.
Life is never simple. This is the point where most people start
adding in safety factors to compensate for some of the unknowns.
These safety factors will almost always guarantee the selection of an
oversized pump that will run off of its
best efficiency point (BEP) most of the
time.

The final calculations are then plotted on the system curve that
describes what the pump has to do to satisfy the requirements of the
application. You can find examples of these calculations in the
alphabetical section of my book and within these web pags. Look
for:

Calculating the total head in metric
units

Calculating the total head in USCS
(inch) units

The pump manufacturer requires a certain amount of
net positive suction head required
(NPSHR) to prevent the pump from
cavitating. He shows that number on his
pump curve. When you look at the curve you will also note that the
net positive suction head required (NPSHR) increases with any
increase in the pump's capacity.

You will also be calculating the net
positive suction head available (NPSHA) to be sure that the
pump you select will not cavitate. Cavitation is caused by cavities
or bubbles in the fluid collapsing on the impeller and volute. In the
pump business we recognize several different types of cavitation.
:

Vaporization cavitation.

Air ingestion cavitation.

Internal recirculation
cavitation.

Flow turbulence cavitation.

Vane Passing Syndrome
cavitation.

Pump cavitation is experienced in several different ways

We can hear cavitation because it sounds like the pump is
pumping ball bearings.

We can see the damage from cavitation on the pump's impeller
and volute.

The operator can sometimes tell if the pump is cavitating
because of a reduction in the pump's capacity.

The main problem with cavitation is that it shakes and bends
the shaft causing both seal and bearing problems. We call all of
this shaking and bending shaft deflection..

Remember that the net positive suction head required (NPSHR)
number shown on the pump curve is for fresh water at 68°
Fahrenheit (20°C) and not the fluid or combinations of fluids
you will be pumping. When you make your calculations for net positive
suction head available (NPSHA) the
formula you will be using will adjust
for your fluid.

In some cases you can reduce the NPSH required. This is
especially true if you are pumping hot water or mixed
hydrocarbons. Look in the alphabetical section under
NPSHR reductions.

You may have to install an
inducer on the pump, add a
booster pump, or go to a
double suction pump design if you do
not have enough net positive suction head available (NPSHA)

When the pump supplier has all of this in-exact information in his
possession he can then hopefully select the correct size pump and
driver for the job. Since he wants to quote a competitive price he is
now going to make some critical decisions:

He might begin with the type of pump he will recommend:

If the capacity were going to be very low he would recommend a
rotary, or positive displacement (PD)
pump.

Between 25 and 500 gpm (5 m3/hr - 115 m3/hr) he will probably
select a single stage end suction
centrifugal pump. It all depends upon the supplier. At
higher capacities he may go to a double suction design with a wide
impeller, two pumps in parallel or
maybe a high-speed pump.

You might need a high head, low capacity pump. The pump
supplier has several options you should know about.

Will he recommend a self-priming
pump? These pumps remove air from the impeller eye and suction
side of the pump. Some operating conditions dictate the need for a
self-priming design. If you do not have a self-priming pump and
you are on intermittent service, will priming become a problem the
next time you start the pump?

How will the pump be operated?

If the pump is going to run twenty-four hours a day, seven
days a week and you are not going to open and close valves; you
will not need a heavy-duty pump.
It is easy to select a pump that will run at its best
efficiency point and at the best efficiency point (BEP) there
is very little shaft displacement and vibration.

Intermittent service is the more difficult application
because of changing temperatures, vibration levels, thrust
direction, etc. Intermittent pumps require a more robust,
heavy-duty design with a low
L3/D4
shaft.

How important is efficiency in
your application? High efficiency is desirable, but you pay a
price for efficiency in higher maintenance costs and a limited
operating window. You should be looking for performance,
reliability, and efficiency in that order. Too often the engineer
specifies efficiency and loses the other two. The following
designs solve some operation and maintenance problems, but their
efficiency is lower than conventional centrifugal pumps.

A magnetic drive or
canned pump may be your best
choice if you can live with the several limitations they
impose.

A vortex or
slurry pump design may be needed
if there are lot of solids or "stringy" material in the
pumpage.

A double volute centrifugal
pump can eliminate many of the seal problems we experience when
we operate off the pump's best efficiency point. The problem is
trying to find a supplier that will supply one for your
application. Although readily available for impellers larger
than 14 inches (355 mm) in diameter they have become very
scarce in the smaller diameters because of their less efficient
design.

The supplier should recommend a
centerline design to avoid the
problems caused by thermal expansion of the wet end if you are
operating at temperatures over 200°F (100°C)?

Will you need a volute or
circular casing? Volute casings build
a higher head; circular casing are used for low head and high
capacity.

Do you need a pump that meets a standard?
ANSI, API, DIN or ISO are some of the
current standards. You should be aware of pump standards problems
that contribute to premature seal and bearing failures. An ANSI
(American National Standards Institute) standard back pullout
design pump has many advantages but presents problems with
mechanical seals when the impeller clearance is adjusted, unless
you are purchasing cartridge seals.

The decision to use either a single or
multistage pump will be determined by
the head the pump must produce to meet the capacities you need.
Some suppliers like to recommend a high speed small pump to be
competitive, other suppliers might recommend a more expensive low
speed large pump to lessen NPSH and wear problems.

There are additional decisions that have to be made about the type
of pump the supplier will recommend:

Will the pump be supplied with a mechanical seal or packing?
If the stuffing box is at negative pressure
(vacuum) a seal will be necessary to
prevent air ingestion.

If he is going to supply a mechanical seal will he also supply
an oversized stuffing box and any
environmental controls that might be needed?

Will he specify a jacketed stuffing
box so that the temperature of the sealed fluid can be
regulated? How does he intend to control the stuffing box
temperature? Will he be using water, steam or maybe a combination
of both? Electric heating is sometimes an option.

How will the open or
semi-open impeller be adjusted to the
volute casing or back plate? Can the mechanical seal face loading
be adjusted at the same time? If not, the face load will change
and the seal life will be shortened.

If the pump is going to be supplied with a
closed impeller you should have some
means of knowing when the wear rings
have to be replaced. If the wear ring clearance becomes too large
the pumps efficiency will be lowered causing heat and vibration
problems. Most manufacturers require that you disassemble the pump
to check the wear ring clearance and replace the rings when this
clearance doubles.

Will he supply a "C" or "D" frame
adapter, or will the pump to motor
alignment have to be done manually
using dual indicators or a
laser aligner to get the readings? A
closed-coupled design can eliminate
the need for an alignment between the pump and driver.

What type of coupling will he
select to connect the pump to its driver? Couplings can compensate
for axial growth of the shaft and transmit
torque to the impeller. They cannot
compensate for pump to driver misalignment as much as we would
like them to. Universal joints are
especially bad because they have to be misaligned to be
lubricated.

He may decide to run two pumps in
parallel operation if he needs a real
high capacity, or two pumps in series
operation if he needs a high head. Pumps that run in parallel or
series require that they are running at the same speed. This can
be a problem for some induction
motors..

An inline pump design can solve
many pipe strain and thermal growth problems.

The pump supplier must insure that the pump will not be
operating at a critical speed or
passing through a critical speed at start up. If he has decided to
use a variable speed drive or motor
this becomes a possibility.

We all want pumps with a low net positive suction head
required to prevent cavitation problems but sometimes it is not
practical. The manufacturer has the option of installing an
inducer or altering the pump design to lower the net positive
suction head required, but if he goes too far all of the internal
clearances will have to be perfect to prevent cavitation problems.
This modification of the impeller to get the low net positive
suction head required (NPSHR) and its affects will be explained
when you learn about suction specific speed.

The difference between specific speed
and suction specific speed can be confusing but you should
know the difference.

Shaft speed is an important decision. Speed affects pump
component wear and NPSH requirements, along with the head,
capacity, and the pump size. High speed pumps cost less initially,
but the maintenance costs can be staggering. Speed is especially
critical if you are going to be specifying a slurry pump.

The ratio of the shaft diameter to its length is called the
shaft L3/D4 number. This ratio will have a
major affect on the operating window of the pump and its inital
cost. The lower the number the better, but any thing below 60 (2
in the metric system) is acceptable when you are using mechanical
seals. A low L3/D4 can be costly in a
standard long shaft pump design because it dictates a large
diameter shaft that is usually found only on expensive heavy-duty
pumps. A short shaft with a smaller outside diameter would
accomplish the same goal, but then the pump would no longer
conform to the ANSI or ISO standard. We often run into
L3/D4 problems when you specify, or the pump
supplier sells you a low cost corrosion resistant sleeve mounted
on a steel shaft rather than a more expensive solid, corrosion
resistant shaft.

There are multiple decisions to be made about the
impeller selection and not all pump
suppliers are qualified to make them:

The impeller shape or specific speed number will dictate the
shape of the pump curve, the NPSH required and influence the
efficiency of the pump.

Has the impeller configuration been
iterated in recent years? Impeller
design is improving with some of the newer computer programs that
have become available to the design engineer.

The suction specific speed number of the impeller will often
predict if you are going to experience a cavitation problem.

The impeller material must be chosen for both chemical
compatibility and wear resistance. You should consider one of the
duplex metals because most corrosion
resistant materials are too soft for the demands of a pump
impeller.

The decision to use a closed impeller, open impeller,
semi-open, or vortex design is
another decision to be made.

Open and semi-open impellers are less likely to clog, but need
manual adjustment to the volute or back-plate to get the proper
impeller setting and prevent internal recirculation.

Vortex pump impellers are great for solids and "stringy"
materials but they are up to 50% less efficient than conventional
designs.

Investment cast impellers are
usually superior to sand cast versions because you can cast
compound curves with the investment
casting process. The compound curve allows the impeller to pump
abrasive fluids with less vane wear.

If you are going to pump low specific
gravity fluids with an open impeller, a non-sparking type
metal may be needed to prevent a fire or explosion. You will be
better off choosing a closed impeller design with soft wear
rings.

The affinity laws will predict
the affect of changing the impeller speed or diameter. You will
want to be familiar with these laws.

Either you or the supplier must select the correct size
electric motor or some other type of
driver for the pump. The decision will be dictated by the specific
gravity of the liquid you will be pumping along with the specific
gravity any cleaners or solvents that might be flushed through the
lines. The selection will also be influenced by how far you will
venture off the best efficiency point (BEP) on the capacity side
of the pump curve. If this number is miscalculated there is a
danger of burning out some electric motors.

How are you going to vary the pump's capacity? Are you going
to open and close a valve or maybe you will be using
a variable speed drive like a
gasoline or diesel engine. Will the regulating valve open and
close automatically like a boiler feed valve or will it be
operated manually? The variable speed motor might be an
alternative if the major part of the system head is friction head
rather than static or pressure head.

The viscosity of the fluid is
another consideration because it will affect the head, capacity,
efficiency and power requirement of the pump. You should know
about viscosity and how the viscosity of the pumpage will affect
the performance of the pump. There are some
viscosity corrections you should make
to the pump curve when you pump viscous fluids.

After carefully considering all of the above, the pump supplier
will select a pump type and size, present his quote and give you a
copy of his pump curve. Hopefully you will be getting his best pump
technology. To be sure that is true you should know what the
best pumping technology is.

At this stage it is important for you to be able to read the pump
curve. To do that you must understand:

Efficiency

Best efficiency point (BEP)

Shut off head.

How to convert pressure to head so you
can reference pump gage readings to the pump curve. When you learn
the three formulas you will get the conversion
information.

Brake horsepower (BHP)

Water horsepower (WHP)

Capacity

Net positive suction head required
(NPSHR)

How to calculate the net positive
suction head available (NPSHA) to the pump to insure you will not
have a cavitation problem.

If all of the above decisions were made correctly the pump
supplier will place his pump curve on
top of your system curve. The required
operating window will fall within the pump's operating window on
either side of the best efficiency point (BEP). Additionally, the
motor will not overheat and the pump should not cavitate.

If the decisions were made incorrectly the pump will operate where
the pump and system curves intersect and that will not be close to,
or at the best efficiency point, producing radial impeller loading
problems that will cause shaft deflection, resulting in premature
seal and bearing failures. Needless to say the motor or driver will
be adversely affected also.

With few exceptions pump manufacturers are generally not involved
in mechanical sealing. You will probably be contacting separate seal
suppliers for their recommendation about the mechanical seal. Recent
mergers between pump and seal companies unfortunately does not
produce the instant expertise we would like sales and service people
to posses.